When teaching accessibility for designers, I usually give the example of the form field error, only shown by red color, to illustrate the "don't convey information only with color".
Some people often go "yeah, it's common sense, none does that anymore, we all at least all put some text next to the error, so, that should be fixed on most websites, right?".

Well, here's an ecommerce form, I used recently:
- nothing explicitly marked as mandatory, yet, all fields are